Shutter lag is a persistent problem for users of digital cameras. Shutter lag is the interval between the time that the camera user indicates that a photograph is to be taken and the time that the camera actually takes the photograph. During the interval, the camera may perform tasks such as automatic focusing and determining optimum exposure settings for the upcoming photograph. These operations take time—typically more time than in a conventional film camera—and the delay may lead to photographer frustration and missed photographic opportunities.
Previous cameras have addressed the problem of shutter lag in various ways. Many cameras use a two-position shutter button. Two states, S1 and S2, are entered sequentially as the shutter button is depressed. The S1 state occurs when the button is partially depressed, and the S2 state occurs when the button is fully depressed. The corresponding shutter button positions are sometimes also called S1 and S2. S1 is an indication that a photograph may be taken imminently, and S2 is an indication that a photograph is to be taken at once. In one previous method, focus and exposure are determined at S1 and “locked”. The locked settings are then used to take a photograph when S2 is reached. This way, a knowledgeable user may focus, wait for the optimum scene composition, and then take a photograph with minimal further delay by pressing the shutter button to S2 because no further focus or exposure calculations are performed. However, this method is of little benefit when there is too little time to focus in advance, and does not help the casual “point and shoot” photographer who may wish to simply aim the camera and fully depress the shutter button expecting a photograph to be taken immediately.
In a second previous method, focus and exposure are continuously updated while the photographer is composing a photo, even before S1 is reached. The photograph may be taken immediately, even if S1 and S2 are traversed in rapid succession, using the most recent updated focus and exposure settings. However, motors used to drive the automatic focus mechanism consume significant electrical energy. Continuous updating of focus and exposure may drain the camera's batteries rapidly, also resulting in user frustration and missed photographic opportunities.
Pending U.S. patent application Ser. No. 10/762,872, by the same inventors as the present application and having a common assignee with the present application, describes a third method for minimizing shutter lag. In this method, sequential preliminary photographs, such as those used for a “live view” shown on a display on the camera, are analyzed to evaluate scene stability. Focus and exposure setting are performed upon each scene change, but are not re-performed while the scene remains stable. When the shutter button is pressed while the scene is stable, a photograph may be taken with minimal delay using the most recent settings. This method conserves battery capacity as compared with the second method above because the focus motors are driven only sporadically.